[server] Add method to get to the main surface from a sub surface

Summary:
Recursively go up to the main surface, that is the top level surface
which doesn't have a parent.

This is useful to know to which surface tree a sub-surface belongs to.

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1207